(a) Field of the Invention
The present invention relates to a manual valve of a hydraulic control system of a continuously variable transmission for vehicles. More particularly, the present invention relates to a manual valve of a hydraulic control system of a continuously variable transmission that minimizes shift shock by controlling release pressure in D→N and R→N shifts.
(b) Description of the Related Art
A continuously variable transmission includes a fixed pulley and a moving pulley. The moving pulley provides thrust force on a side surface of a metal belt to a degree suitable for driving torque by hydraulic pressure in a hydraulic pressure chamber at a rear portion of the moving pulley. Continuous shifting occurs as a result of changes in diameter of the pulleys.
A continuously variable transmission also includes a planetary gear set with three operational elements connected by a forward clutch and a reverse clutch such that two of the three operational elements always operate as an input and an output element, and the third selectively acts as an input element or a fixed element.
Since the operating pressure supplied to the forward clutch and the reverse brake is exhausted only through an exhaust port formed in a manual valve, active control of exhaust hydraulic pressure is impossible.
However, viscosity of transmission fluid changes depending on temperature. Exhaust time is thus shorter at room temperature, and longer at lower temperatures because of an increase of viscosity of the transmission fluid. Exhaust time is traditionally not controlled in response to temperature, so shift shock occurs in D→N and R→N shifts.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.